Updated the coveralls badge without a branch name

Author: Takashi Matsuo

README.md

@@ -3,7 +3,7 @@
 This repository is holds the samples used in the python documentation on [cloud.google.com](cloud.google.com).
 
 [![Build Status](https://travis-ci.org/GoogleCloudPlatform/python-docs-samples.svg?branch=master)](https://travis-ci.org/GoogleCloudPlatform/python-docs-samples)
-[![Coverage Status](https://coveralls.io/repos/GoogleCloudPlatform/python-docs-samples/badge.svg?branch=master)](https://coveralls.io/r/GoogleCloudPlatform/python-docs-samples?branch=master)
+[![Coverage Status](https://coveralls.io/repos/GoogleCloudPlatform/python-docs-samples/badge.svg)](https://coveralls.io/r/GoogleCloudPlatform/python-docs-samples)
 
 For more detailed introduction to a product, check the README in the corresponding folder.